Protocol for the synthesis of perovskite nanocrystal thin films via in situ crystallization method

Summary The field of halide perovskites currently faces the challenge of finding an efficient approach for producing highly efficient and stable perovskite nanocrystals (PNCs). Here, we present a protocol for the annealing-free and antisolvent-free synthesis of PNCs. We describe the steps for preparing the PNCs precursor solutions. We then detail the procedures to control crucial processing parameters, such as the role of precursor concentration and the creation of humidity-controlled chambers, which allow achieving precise control over the final nanocrystals size. For complete details on the use and execution of this protocol, please refer to Noguera-Gómez et al.1

The whole described protocol must be carried out under the appropriate safety conditions regarding each chemical and its derived subproducts (e.g., laboratory facilities, individual protection, and suitable waste treatment).Specifically, lead is a toxic metal that can have detrimental effects on human health.Exposure to lead, whether through inhalation, ingestion, or skin contact, can lead to various health issues.The central nervous system, kidneys, blood, and reproductive system are particularly vulnerable to lead poisoning. 10To ensure safety when working with lead-based materials and solvents such as N,N-dimethylformamide (DMF), while following this protocol, it is crucial to follow several precautions.First and foremost, one should always wear personal protective equipment (PPE), such as gloves, goggles, and a respirator, to minimize direct contact and inhalation of lead particles.Work areas should be well-ventilated to prevent the accumulation of lead dust.To ensure safe handling, use, storage, and disposal of the chemical/product as hazardous waste, it is essential to develop a Standard Operating Procedure (SOP) and/or conduct a Job Hazard Analysis (JHA).This procedure should include comprehensive guidelines for handling, storage, and disposal procedures, along with the PPE.Additionally, it is crucial to train workers on reagents hazards, provide them with Safety Data Sheets (SDSs), familiarize them with SOPs and JHAs, and ensure proper labeling of chemicals and lead products to promote clear identification and minimize potential risks.By adhering to these safety instructions, individuals can significantly reduce the potential health hazards associated with lead exposure.Note: All the chemical reagents can be obtained from different providers as long as they keep the purity degree.

MATERIALS AND EQUIPMENT
Given example of stock solutions for the PNCs nanocomposite thin films.
Note: It is preferable to prepare the final stock solution of the nanocomposite once for each experiment to avoid premature aging caused by the precursors' interaction in the solution.

STEP-BY-STEP METHOD DETAILS
Solution and material preparation (precursor solution) This section outlines the preparation of the two primary precursor solutions.These solutions are meticulously mixed to achieve a transparent solution, which serves as the desired outcome and is then ready for the subsequent deposition step.
We consider MAPbBr 3 :Ni(AcO) 2 with a 0.25:1 M ratio for the reference case scenario.For the preparation of that ratio.ii.Once finished, filter the solution using a 5 mL syringe with a 0.2 mm PVDF filter.2. Preparation of MAPbBr 3 perovskite precursor solution.
a. Prepare a 2 mL 0.5 M MAPbBr 3 solution: i.In one vial, dissolve MABr 1 mmol (0.112 g) in 2 mL of DMF by stirring for 20 min.
ii.From the previous vial, take 1 mL of the solution and add it to a second vial containing 1 mmol (0.367 g) of PbBr 2 .iii.Filter the solution from step (ii) using a 5 mL syringe with a 0.2 mm PVDF filter.Store the filtered solution at room temperature approx.20 C and 40% (RH).Note: Experiments exploring different ratios to obtain varying particle sizes were conducted, primarily focusing on various ratios MAPbBr 3 by diluting with DMF.In this experiment, the Ni(AcO) 2 matrix concentration was maintained at 1 M (see Figure 1).

CRITICAL:
The matrix and perovskite solutions exhibit long-term stability, maintaining their integrity and properties for up to 3 months when stored at normal laboratory conditions (approx.20 C-50% RH).

Substrates cleaning
Timing: 1 h In this section, we detail the steps involved in ensuring the cleanliness of the substrates prior to their use in the deposition step.Thorough cleaning procedures, including solvent cleaning, rinsing, and drying, are implemented to eliminate any unwanted particles or residues.By effectively cleaning the substrates, we aim to provide a pristine surface for optimal adhesion and wettability of the surface.b.Clean with a toothbrush and 2% Hellmanex solution in water.c.Immerse in an ultrasonic bath containing a 2% Hellmanex (supplier) solution in water for 30 min.d.Rinse thoroughly with deionized water.e. Rinse with ethanol (supplier).f.Immerse in an ultrasonic bath containing IPA and Acetone (supplier) in a 3:1 ratio for 15 min.g.Dry the substrates using moisture and dust-free compressed air.

Timing: 5-30 min
The deposition with a spin coater enables the efficient removal of the excess of solution and drying of solvents, while also facilitating the formation of a thin film with the desired properties adjusting the spinning parameters.
CRITICAL: As the process strongly depends on humidity, the deposition can be carried out either inside a dry-glovebox (SICCO) supplied with moisture-free air or under ambient laboratory conditions (approx.40-50% RH and 20 C).The final size of the synthesized PNCs (Figure 2) will be determined by the RH chosen for the deposition process.
Note: With this procedure, the resulting film thickness is in the range of 300 nm.Although, the spin-rate primarily controls the film thickness that influences the absorption properties, while the final size of the PNCs is less influenced by the spin-rate and more dependent on other factors.

Depositing within a dry box
CRITICAL: All the necessary instruments must be placed within this dry box for at least 20 min before the deposition and under moisture-free air.

Protocol
The single-step synthesis process only consists of depositing the nanocomposite precursor solution onto a glass.5. Operate with the spin-coater as described: a. Center the glass substrate (treated with ozone in the spin-coater and set the operating parameters to 3000 rpm for 40 s with an acceleration ramp of 2000 rpm/s.b.Drop 70 mL of the solution in the center of the glass substrate in static using a Micropipette and start the spin-coater program. Once the program is finished, place the sample within a previous-fabricated humidity chamber (as described below in the humidity chambers building subsection) and for the appropriate time to trigger the crystallization with specific PNC size (see Noguera-Go ´mez et al. 2022).
Depositing in standard laboratory conditions CRITICAL: Depositing outside of a dry box will propitiate the crystallization of the PNCs when exposed to humidity conditions with a RH difference > 10%) (see Figure 6).
Note: In this scenario, please perform the steps outlined in the previous instructions but conduct the procedure outside the dry box.The relative humidity and temperature (laboratory conditions) during the deposition step will influence the final size of the PNCs dramatically.

Humidity Chambers Building
Timing: 1 h In this section we provide an overview of the design and construction of the humidity chambers used to generate precise humidity levels ranging from 10 to 100% RH. 11 Humidity chambers have been designed to generate a specific humidity level from 10 to 100% RH by using a supersaturated solution of different inorganic salts.The salt solutions are used because they have a well-defined equilibrium vapor pressure at a given temperature, which makes it possible to generate a specific level of relative humidity inside a chamber.
The humidity chambers consist of 4 essential parts (Figure 3): Polypropylene Box with an airtight lid.

Thermo-Hygrometer.
CRITICAL: To guarantee the desired relative humidity level, once the supersaturated salt solution is placed in the chamber, the chamber is closed with an airtight lid.The RH is monitored with a hygrometer.The equilibrium vapor pressure is reached after 30 min for a 20 3 20 3 20 cm box.Plastic trays were designed and attached to one of the side walls of the chamber to enable the insertion of samples without breaking the equilibrium.The trays (Data S1-STL Printable File -Sample Tray Holder for Humidity Chambers) were 3D printed using polylactic acid (PLA) with a 100% infill.STAR Protocols 4, 102507, December 15, 2023

Protocol
Note: Adding multiple trays does not negatively impact the establishment of equilibrium within the vessel.
The relative humidity is determined by the partial vapor pressure of the salt solution mixture at a given temperature once the equilibrium is reached.To prepare a humidity chamber, we use different inorganic salts available according to the desired RH (see Table 1).
6. Preparation of saturated salt solutions involves the following steps: a. Select the appropriate salts based on the RH range being investigated (Table 1).b. Fill the bottom of the chamber with 1 cm layer using the salt from step 1. c.To achieve this, slowly add small portions of distilled water while gently agitating to promote a homogeneous distribution with the salt.The final goal is to establish a slushy mixture that ensures a supersaturated state of the salt in aqueous solution.d.Upon reaching the supersaturated state, close both the airtight lid and the box trays and wait approximately 30 min to reach the equilibrium vapor pressure (check the RH with a thermo-hygrometer).
CRITICAL: The minimum recommended box size is 20 3 20 3 20 cm.
Note: Table 1 compiles common inorganic salts as potential candidates for preparing controlled humidity chambers.As previously mentioned, in a state of supersaturation, the containers establish a vapor equilibrium that enables the attainment of relative humidity at the tabulated temperature.

EXPECTED OUTCOMES
We have developed a method that enables the synthesis of PNC nanocomposites through a novel in situ approach, which allows controlling the final particle size.Compared with similar methods, our approach is a facile antisolvent-free and glovebox-free (cost-effective) strategy with excellent emission properties.
To effectively characterize thin films, both X-ray diffraction (XRD) and transmission electron microscopy (TEM) can be studied to accurately analyze the formation of the PNCs nanocomposites.These techniques allow for assessing the existence of PNCs in the matrices following the prosperous production of PNCs nanocomposites.This evaluation is depicted in Figure 4. Note: The in situ synthesis of MAPbBr 3 in Ni(AcO) 2 can be confirmed by the X-ray diffraction (XRD) diffractogram (Figure 4A), which reveals diffraction peaks corresponding to the cubic Pm3m crystal phase of MAPbBr 3 (JCPDS no.00-0105) in the nanocomposite.Notably, films prepared under RH <10% exhibit no crystalline patterns, indicating that MAPbBr 3 does not form at low RH.This finding is consistent with the negligible optical properties observed in the absorbance and PL spectra (Figure 6C).However, upon exposure of the films deposited at RH <10% to the ambient atmosphere (RH >15%), the crystallization process initiates, resulting in a gradual color change and an increase in the excitonic absorption band and PL.
For the film crystallization processes under the same RH, higher perovskite precursors concentration leads to larger PNC size, as confirmed by transmission electron microscopy (TEM) measurements (Figures 4B and 4C).At a lower concentration of MAPbBr 3 (0.25 M), smaller PNC sizes are observed, with a distribution centered around 2-3 nm, while larger amounts of MAPbBr 3 (0.5 M) result in larger PNCs, with a distribution centered around 6-7 nm.
The role of concentration in the formation of the PNCs nanocomposite is crucial.Therefore, when the concentration ratio of the PNCs precursor is varied with respect to the matrix, the expected outcome can be studied as depicted in Figure 5.After a successful synthesis, the optical properties of the resulting thin films fabricated at 50% RH are characterized (absorption and PL spectra).The equipment needed for these characterizations can be found in the key resources table.
Note: As greater amounts of MAPbBr 3 are loaded within the matrix (Figure 5A), up to the bulk phase (1:0), the absorption spectra (Figure 5B) show a clear trend of increasing absorption peaks that shift towards lower energies.In addition, the photoluminescence (PL) response (Figure 5C) depicts a redshift, transitioning from wavelengths ranging from 515 nm in the sample with a lower amount of MAPbBr 3 (0.25:1 M) to 550 nm in the pure bulk MAPbBr 3 sample.This shift is attributed to the variation in average particle sizes within the matrix (as the TEM Images supported in the previous subsection) with funneling effects. 1,12ternatively, to the precursors' concentration effect, RH also impacts the crystallization dynamics.
The PL and absorption spectra of perovskite nanocomposite films with a MAPbBr 3 :Ni(AcO) 2 ratio of 0.25:1 M are shown in Figure 6.These spectra are measured after exposing the films to various RH levels for 60 min.The absorption edge and emission peak exhibit a redshift as RH increases from 50% to 100%.The most prominent distinction between the two absorption spectra is the clear excitonic absorption peak at 525 nm when the film is exposed to a RH of 100%, unlike the one exposed to 50%.Furthermore, when exposed to higher RH, a PL band shift is observed from 525 nm (RH of 50%) to 540 nm (RH of 100%).These results are in perfect accordance with the TEM Images discussed above.
CRITICAL: It is noteworthy that while ambient humidity affects the crystallization process, the optical properties of the nanocomposite film are stable once a crystallization plateau is reached.The stable conditions can be reached after different periods of exposure to certain RH conditions for each precursor concentration.
Thus, it becomes essential to understand the crystallization dynamics to tune the optoelectronic properties of the perovskite thin film nanocomposites in a stable way.While the exposure to higher ambient humidity results in faster perovskite crystallization, the precursor concentration can also affect this reaction.The impact of an RH of 50% is investigated on two different samples with different precursors concentration immediately after preparation within a dry-ambient glovebox.
The in situ synthesis of PNCs within the Ni(AcO) 2 is monitored by their optical properties at different times, as shown in Figure 7.Our observations indicate that the reaction is more abrupt when the perovskite concentration is higher (0.5 M), working with the matrix fixed at a concentration of 1 M.This phenomenon may occur due to the existence of more nucleation centers in the medium, which is derived from the higher perovskite concentration.Besides, a plateau of emission is observed at shorter times for higher concentrations (0.5 M), which is related to an almost complete reaction of the perovskite precursors within the Ni(AcO) 2 matrix.

LIMITATIONS
One of the limitations is that precursor solutions, especially with sol-gel precursors such as the Ni(AcO) 2 , may eventually precipitate after several months.Hence, the filtration step plays a critical role in ensuring the maximal longevity of the precursor by removing unsolved particles that can act as nucleation centers.

TROUBLESHOOTING
The approach of synthesizing PNC nanocomposites in situ can potentially establish a foundation for producing optoelectronic devices on a large scale with improved properties.It can also serve as a framework for tuning the bandgap through the crystallization dynamics control of the PNC size.
To ensure successful synthesis of the PNC nanocomposites using the in situ approach, it is important to be aware of potential issues that may arise during the process.The following troubleshooting section aims to address common challenges and provide possible solutions.
Problem 1 -Particle Agglomeration Aggregation, sudden aging, or clustering of the sol-gel matrix solution, leading to poor dispersion and heterogeneous uniformity (solution and material preparation (precursor solution) Subsection).

Potential solution
If any of the solutions (MAPbBr 3 or Ni(AcO) 2 ) unexpectedly precipitates, it is advisable to replace it with a fresh one, even if it was expected to have a longer shelf life of three months.This ensures the reliability and consistency of the solution's properties for the next steps.
Problem 2 -Solutions preparation Undissolved reagents at the bottom of the vial after following the instructions for the preparation of Ni(AcO) 2 (solution and material preparation (precursor solution) Subsection).

Potential solution
To ensure optimal homogeneity and complete solubilization of the solution, gently heat the mixture while stirring in the dry-block bath.This will promote better mixing and aid in the dissolution process.If needed increase the heating temperature of the dry-block bath up to 80 C and heat for 5 additional min.Once done, proceed with filtration to remove any remaining particulates or undissolved components.

Figure 1 .
Figure 1.Schematic representation of the nanocomposite fabrication process: an insight into the impact of the concentration and the RH on the PNCs crystallization MAPbBr 3 :Ni(AcO) 2 solution.a. Mix 1 mL of step two solution with 1 mL of step one.b.Stir the vial containing the mixture for 10 min.c.Store the filtered solution at room temperature approx.20 C and 40% (RH).

Figure 3 .
Figure 3. Humidity Chamber (A) Real-built Humidity Chamber.(B) Schematic representation of a Humidity Chamber.

Figure 5 .
Figure 5. PNCs nanocomposite thin films (A) Photograph of the PNCs nanocomposite thin films at different concentration ratios of MAPbBr3 (Ni(AcO)2 was fixed to 1 M).(B and C) Absorption and PL Spectra of the thin films, respectively.

Figure 6 .
Figure 6.Humidity Influences (A and B) Photographs of nanocomposite thin films under different RH conditions, <10% and >50%, respectively.(C) PL and absorption spectra of the nanocomposite film fabricated with a MAPbBr3:Ni(AcO)2 ratio of 0.25:1 M and crystallized under varying RH conditions.

Table 1 .
Inorganic salts for the generation of different RH atmospheres within the Humidity Chambers